1. Field of the Invention
The present invention relates to a motor-driven supercharger having an electric motor built-in.
2. Description of Related Art
To supply an air having a boosted density by a compressor to an engine is called as supercharging, and an apparatus providing for a driving work of the compressor on the basis of an exhaust energy is called as an exhaust gas turbine supercharger (an exhaust-gas turbocharger).
FIG. 1 is a view showing one example of a conventional exhaust gas turbine supercharger. As shown in this drawing, the conventional exhaust gas turbine supercharger is generally constituted by a compressor 41 and a turbine 42 which are arranged so as to provide a bearing unit 40 therebetween, and the compressor 41 and the turbine 42 respectively have a compressor impeller 41a and a turbine impeller 42a built-in. The compressor impeller 41a and the turbine impeller 42a are coupled to each other by a connecting shaft (a shaft 43) supported by a bearing unit 40, and are structured such that the turbine impeller 42a is rotationally driven by an exhaust gas of the engine, a rotating force is transmitted to the compressor impeller 41a via the shaft 43, and the air is compressed by the compressor impeller 41a so as to be  supercharged to the engine.
Further, in this drawing, the bearing unit 40 has a radial bearing 44 supporting a radial force of the shaft 43 and a thrust bearing 45 supporting a thrust force. The radial bearing 44 is constituted by a full-floating bearing in this example, however, there is a case that it is constituted by a semi-floating bearing.
The thrust bearing 45 is constituted by a disc-shaped thrust collar 46 rotating together with the shaft 43, and a turbine side thrust bearing 47 and a compressor side thrust bearing 48 which block a movement in an axial direction of the thrust collar 46.
On the other hand, in the exhaust gas turbine supercharger mentioned above, there has been already proposed a structure having an electric motor built-in for assisting an acceleration at a time of rotating at a low speed (for example, patent document 1). Hereinafter, the exhaust gas turbine supercharger having the electric motor mentioned above built-in is simply called as “motor-driven supercharger”.
The turbocharger in the patent document 1 is provided with an electric motor (a stator 51 and a rotor 52) rotationally driving a rotor, as shown in FIG. 2. The rotor is constituted by a turbine impeller 53, a compressor impeller 54, and a spacer collar 56 to which a shaft 55 connecting the impellers and the rotor 52 of the electric motor are attached. The shaft 55 is supported by  a single semi-floating bearing 57. The semi-floating bearing 57 has thrust surfaces 57a and 57b engaging with a hub 53a of the turbine 53 and the spacer collar 56.
In accordance with this structure, the thrust force applied to the rotor is supported by the thrust surfaces 57a and 57b engaging with the hub 53a of the turbine and the spacer collar 56 in the semi-floating bearing 57.
Patent Document 1: U.S. Pat. No. 6,449,950 “ROTOR AND BEARING SYSTEM FOR ELECTRICALLY ASSISTED TURBOCHARGER”
In the exhaust gas turbine supercharger, since a pressure applied to the turbine impeller is generally higher than a pressure applied to the compressor impeller, a great thrust force is generated in a direction toward the compressor impeller.
Accordingly, it is necessary to supply a sufficient amount of lubricating oil to the thrust bearing supporting a rotating shaft of the exhaust gas turbine supercharger.
However, in the case that the thrust bearing is provided in an outer side of the radial bearing, and the electric motor is provided between the thrust bearing and the compressor impeller, there is a problem that an overhang amount of the shaft is enlarged at a length of the thrust bearing and the electric motor.
For example, in the prior art in FIG. 1, if the stator and the rotor of the electric motor are provided between the compressor impeller 41a and the seal plate 49, it is necessary to elongate the overhang amount of the  shaft at least at an amount corresponding to the stator of the electric motor, and there is a risk that a rotation stability is lowered at a time of rotating at a high speed.
Further, in the prior art in FIG. 2 having the stator and the rotor of the electric motor built-in, since a distance between the thrust surfaces 57a and 57b in both ends of the semi-floating bearing 57 is long, there is a problem that a gap (a play) tends to be generated in a thrust direction due to a heat expansion or the like. Further, in this example, it is hard to supply and discharge a sufficient amount of lubricating oil to the thrust surface 57a suffered to the great thrust force in the direction toward the compressor impeller.